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1-Stage Freedom analysis

Calculate cluster-level sensitivity and specificity for range of sample sizes and cut-points for given cluster size and imperfect tests

Sample size range:

Design prevalence values (enter up to 5 values, as proportions):

Cut-point range:

Method of calculation

This utility calculates cluster sensitivity (SeH) and cluster specificity (SpH) for a range of sample sizes and cut-point numbers of reactors, for specified cluster size, test sensitivity, test specificity and design prevalence values. Options for calculations include four different estimation methods:

  • Binomial distribution: where cluster size is not specified;
  • HerdPlus: Using the hypergeometric distribution;
  • HerdAcc: Jordan D, McEwen SA, 1998. Herd-level test performance based on uncertain estimates of individual test performance, individual true prevalence and herd true prevalence. Prev. Vet. Med. 3: 187-209; and
  • FreeCalc: Cameron AR, Baldock FC, 1998. A new probability formula for surveys to substantiate freedom from disease. Prev. Vet. Med. 34: 1-17.

Inputs include:

  • Test sensitivity and specificity;
  • Specified cluster/population size;
  • Maximum and minimum sample sizes to be considered and the step in values;
  • Up to 5 unit-level design prevalences (the hypothetical prevalence to be detected, as proportions);
  • Maximum and minimum cut-point number of positives to classify a cluster (herd/flock) as positive;
  • The preferred calculation method; and
  • The desired precision of results (number of digits to be displayed after the decimal point).

Outputs include:

  • Tables and plots of SeH and SpH for each design prevalence value for combinations of sample size and cut-points; and
  • An excel file of the results.